This invention relates to pan granulation of low-water or substantially anhydrous nitrogenous products from pure or mixed melts of compounds selected from the group consisting of urea, ammoniumnitrate and -phosphate, with or without other compounds added. These products are primarily utilised as fertilizers, and it is essential that they are available in the form of strong and storable granules with good spreadability.
During pan granulation the granules are gradually built up by spraying or spreading a liquid phase against a rolling bed of material on a circular, rotating and inclined pan onto which is charged comparatively finely divided solids as coolant. The effect of using a pan implies also that the product, i.e. the granules on the pan, are classified according to size. As the particles increase in size, they will gradually move upward in the bed and outward in the direction of the periphery of the pan in such a way that when the granules have attained the adequate size they will roll over the pan edge on that part of the pan where the particles are being lifted by the rotation of the pan. Smaller particles will be held back on the pan and grow till they become large enough to roll over the pan edge. By running the pan correctly, the particles which leave the pan will be of fairly uniform size. The particle size is mainly a function of the pan's rate of rotation and angle of inclination.
The amount of material of the rolling bed is not uniformly distributed over the pan area. The largest amount of material is on the upward moving part where the particles roll over the pan edge. Here, in the opposite direction of the lifting movement of the pan, a continuous sliding action takes place. The material depth decreases the direction of the diametrically opposite edge.
Pan granulation techniques were orginally developed for the conversion of dry pulverant and finely divided material, during the addition of moisture, usually water, to larger spherical granules or pellets, the granules are predominately formed by cementing together -- or agglomeration of individual particles.
Granulation by agglomeration is a satisfactory techniques when using free flowing pulverant material, for instance for production of pellets for metallurgical purposes, dressing of minerals, ores, etc. with water or other low viscosity liquid binders. Pan granulation of substantially anhydrous melts, for instance fertilizers, has formerly been tried, but with poor results. By such processes it is desireable to produce fairly small particles, and it is important with regard to mixability, segregation, spreadability etc. that the particles are strong and of approximately uniform size.
By using the above mentioned melts of fertilizers it has so far been impossible to maintain satisfactory classification simultaneously with high growrate and dense particle structure.
The mobility of the individual particles will be decreased if the circulating mass of solids is heavily moistened by warm melt. This reduces the classification and sorting mechanism by the fact that free slides are obstructed and pulsating currents will occur, which leads to uncontrolled growth and overflowing of not completed particles over the rim of the pan. Simultaneously there will be a reagglomeration of too sticky, single particles forming large, warm aggregates which get too hot and disintegrate to a warm, sticky, crystalline mass. Accordingly the material will accumulate on the pan and the process breaks down. In order to avoid such problems and obtain a satisfactory granulation of such melts, the agglomeration techniques have been dropped and one has turned to granulation at low temperatures and applying other special conditions to secure a fast and complete solidification of the melt sprayed on the single particles. Strong spherical granules, built up of concentric layers of solidified melt, are obtained by using this technique.
U.S. Pat. No. 3,117,020 describes a method of pan granulation of substantially anhydrous solutions of urea and ammonium nitrate, where the solution has a predetermined water content of 5-8 weight percent. The granulation is carried out under conditions resulting in evaporation of water as the solution is being sprayed on the moving particles of the bed, which thereby are kept at a sufficiently low temperature to insure that the added layer solidifies immediately. The upper concentration limit of the solution is 95 weight percent and it is mentioned in this patent that a lower water content than 5 weight percent gives such a reduction in heat removal that the rolling particles of the bed are converted to a sticky mass impossible to handle.
U.S. Pat. No. 3,408,169 described a method of pan granulation of melts of urea and ammonium nitrate. According to this patent anhydrous urea or ammonium nitrate melt is sprayed onto a specially formed zone of fast moving cooled particles. This zone is positioned substantially diametrically opposite to a thicker bed of a crescent shape comprising slower moving, rolling particles on the discharging part of the pan. In this zone there is a relatively thin, densely packed layer of small particles mixed with cooled solids charged on the pan. The small particles follow the pan during its rotation while the cooled solids are charged onto the outmost upper part of the pan and mix with the warmer small particles just before the hot melt is sprayed on the pan. Hereby occurs a rapid cooling and solidification of the melt, whereby uncontrolled agglomeration is avoided. Dense granules are built up having an onion-like structure and consisting of several concentric layers of solidified melt.
In spite of the fact that the problems in pan granulation of substantially anhydrous and ahydrous nitrogenous products have been solved, these known methods using layering techniques for making granules, constituting layers of solidified melt, have not made a break-through industrially compared to rotary drum granulation and prilling. This is primarily due to too low an output of the pan by such low-temperature techniques where solidification of the melt in layers dominates the particle growth and where net pan output of 500-800 kg/m.sup.2 h is considered very high.
The literature gives by example the production capacity of a pan for granulating urea, ammoniumnitrate and ammoniumnitrate/-calciumcarbonate to be 15, 6 and 8 ton/m.sup.2 day, respectively, and this is considered to be the natural upper production limit for one unit.